[PATCH v3 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs

[Nitesh Narayan Lal <nitesh@redhat.com>]

If we have isolated CPUs dedicated for use by real-time tasks, we try to
move IRQs to housekeeping CPUs from the userspace to reduce latency
overhead on the isolated CPUs.

If we allocate too many IRQ vectors, moving them all to housekeeping CPUs
may exceed per-CPU vector limits.

When we have isolated CPUs, limit the number of vectors allocated by
pci_alloc_irq_vectors() to the minimum number required by the driver, or
to one per housekeeping CPU if that is larger.

Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
---
 include/linux/pci.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/pci.h b/include/linux/pci.h
index 835530605c0d..a7b10240b778 100644
--- a/include/linux/pci.h
+++ b/include/linux/pci.h
@@ -38,6 +38,7 @@
 #include <linux/interrupt.h>
 #include <linux/io.h>
 #include <linux/resource_ext.h>
+#include <linux/sched/isolation.h>
 #include <uapi/linux/pci.h>
 
 #include <linux/pci_ids.h>
@@ -1797,6 +1798,22 @@ static inline int
 pci_alloc_irq_vectors(struct pci_dev *dev, unsigned int min_vecs,
 		      unsigned int max_vecs, unsigned int flags)
 {
+	unsigned int hk_cpus;
+
+	hk_cpus = housekeeping_num_online_cpus(HK_FLAG_MANAGED_IRQ);
+	/*
+	 * If we have isolated CPUs for use by real-time tasks, to keep the
+	 * latency overhead to a minimum, device-specific IRQ vectors are moved
+	 * to the housekeeping CPUs from the userspace by changing their
+	 * affinity mask. Limit the vector usage to keep housekeeping CPUs from
+	 * running out of IRQ vectors.
+	 */
+	if (hk_cpus < num_online_cpus()) {
+		if (hk_cpus < min_vecs)
+			max_vecs = min_vecs;
+		else if (hk_cpus < max_vecs)
+			max_vecs = hk_cpus;
+	}
 	return pci_alloc_irq_vectors_affinity(dev, min_vecs, max_vecs, flags,
 					      NULL);
 }
-- 
2.18.2